In a study group of 2,457 consecutive patients undergoing cardiac catheterization, 30 patients had coronary arterial ectasia, an irregular dilatation of major vessels up to seven times the diameter of branch vessels. The frequency of hypertension, abnormal electrocardiogram and history of myocardial infarction was greater than that in a control group with obstructive coronary artery disease. Patients with ectasia did not differ from patients with obstructive disease in sex, age, prevalence of angina or presence of metabolic abnormalities. Six deaths occurred in the group with ectasia during a mean follow-up period of 24 months (annual rate of 15 percent). Extensive destruction of the musculoelastic elements was evident, resulting in marked attenuation of the vessel wall. The short-term prognosis in this group is the same as in medically treated patients with three vessel obstructive coronary artery disease.